I ran into this problem when trying to build an App for iOS on my Mac.

I use the FileDialog of the Qt.labs.platform 1.0 on Qt 5.9.1, which works absolutely fine on my Desktop builds for MacOS, Linux and Windows. According to Documentation it should also work for iOS.

But my App for iOS kept crashing during the initialization of the FileDialog stuff, so I tried a simple example with 

import QtQuick 2.9
import QtQuick.Controls 2.2
import Qt.labs.platform 1.0

Item {
    FileDialog {
        id: fileDialog
        folder: StandardPaths.writableLocation(StandardPaths.DocumentsLocation)
    }
    Button {
        id: button1
        text: qsTr("Press Me")
        onClicked: {
            fileDialog.open()
        }
    }
}

which does absolutely nothing, not even issue a warning, on the iOS Simulator as well as on the devices (iOS 9 and 10 Xcode 8.3.3)

When using the  import QtQuick.Dialogs 1.2 instead, it works on the Simulator but does complain on the Device with

qrc:/test.qml:4 module "QtQuick.Dialogs" is not installed